1. Field of the Invention
The present invention relates to a flash lighting circuit which is installed in a camera and emits a flashlight in photography.
2. Description of the Prior Art
Most of recently developed cameras are provided with a flash lighting circuit capable of photographing on a film of a specified sensitivity even when a brightness of a subject to be photographed is small, for example, at night. Electronic cameras using such flash lighting circuit for automatic exposure control have been put into practical use which measures a brightness of the subject with flash lighting immediately before photographing, sets exposing conditions such as a diaphragm value, shutter speed, etc. based on the result of measurement, and exposes the film by another flash lighting under the above described exposure conditions. Hereafter flash lighting before photographing is referred to as "preliminary lighting" and flash lighting in photographing is referred to as "main lighting".
The flash lighting circuit of such electronic camera is provided with a small capacity sub capacitor SC for preliminary lighting and a large capacity main capacitor MC for main lighting, as shown in FIG. 1, to quickly shift the operation to the main lighting by reducing a charging time after preliminary lighting. Only the sub capacitor SC is discharged to make preliminary lighting, the sub capacitor SC is immediately charged, and both the sub capacitor SC and the main capacitor MC are discharged to make main lighting and then charged for next flash photography.
The following describes in detail the above lighting operation. For preliminary lighting, the lighting changeover thyristor SCR1 is kept off with a lighting changeover signal (MSCR) and a circuit between the main capacitor MC and a flash discharge tube Xe in which xenon gas is sealed is disconnected. When the trigger thyristor SCR2 is turned on with a trigger signal (TRG) under this condition, a charge stored in a trigger capacitor TC flows into a trigger transformer TT, a high voltage is produced at the secondary side of the trigger transformer TT and applied to the trigger electrode provided on the wall of the flash discharge tube Xe to activate xenon gas in the discharge tube and the resistance of the discharge tube is reduced. In this case, since the circuit between the main capacitor MC and the flash discharge tube Xe is kept disconnected, only charge stored in the sub capacitor SC is discharged and a flash is emitted. This emission is the preliminary lighting.
On the other hand, for main lighting, the lighting changeover thyristor SCR1 and the trigger thyristor SCR2 are turned on at the same time. Then, both the main capacitor MC and the sub capacitor SC electrically connects to the flash discharge tube Xe and the charge stored in both capacitors is discharged into the flash discharge tube Xe to make flash lighting.
Charging is done as described below. Specifically, the DC-DC converter CT boosts a power supply voltage (6V) from a battery E according to a charging control signal (charging start signal START) and applies a high voltage (250V) to the main capacitor MC and the sub capacitor SC to charge these capacitors. When the main capacitor MC and the sub capacitor SC are charged up to the rated capacity, a Zener current begins to flow in the Zener diode ZD and is outputted as a charging detection signal (FULL) to the control part, not shown, through a resistor R1. The control part outputs the charging control signal (charging stop signal STOP) to the DC-DC converter in accordance with the charging detection signal to stop charging.
Charging of the sub capacitor SC is enabled with the charging resistor R2. Specifically, if the charging resistor R2 is not provided, the circuit between the sub capacitor SC and the DC-DC converter CT is disconnected with the lighting changeover thyristor SCR1 and therefore the sub capacitor SC cannot be charged. In this case, the circuit between the sub capacitor SC and the DC-DC converter CT is connected with the charging resistor R2 so that a high voltage of the DC-DC converter CT can be applied to the sub capacitor SC through the charging resistor R2.
Lately, electronic cameras each being provided with the flash lighting circuit has also tendency to be made in an extremely compact design. However, if the camera is made compact, distance between the flash lighting window and the photographing lens is short and therefore, when a flash light enters into eyes of a subject person in flash lighting photography, the flash light is reflected as red light from capillary vessels on the retina and the so-called red eyes phenomenon that pupils in color photography appear in red will often occur.
For this reason, electronic cameras which are intended to reduce such red eyes reflection have been materialized which are adapted to cause size of the pupils to be instantaneously made smaller upon preliminary lighting by the flash lighting circuit and make the main lighting before the pupils recover their former size. Currently, the preliminary lighting has been demanded to be commonly used both in automatic exposure control and reduction of red eyes.
However, the conventional flash lighting circuit which is primarily constructed to perform preliminary lighting for automatic exposure control requires more charging time because the sub capacitor SC needs to be charged through the charging resistor R2. Therefore the charging of the sub capacitor SC cannot be finished within an appropriate duration of time (0.7 to 1.3 seconds) to reduce the red eyes reflection, the main lighting is carried out with lapse of a longer time than the above, the pupils open large at the time of main lighting and the red eyes reflection could not be fully reduced. Particularly, if the power supply voltage falls, the charging time will be further longer.
For main lighting by the main capacitor MC and the sub capacitor SC, the sub capacitor SC of a small capacity is discharged up earlier than the main capacitor and, when the resistance value of the sub capacitor SC becomes approximately 0.OMEGA., a charging current from the main capacitor MC begins to flow in the sub capacitor SC and the lighting changeover thyristor SCR1. The charging current at the initial moment of flowing is extremely large and therefore the lighting changeover thyristor SCR1 should be a large and expensive type which substantially hinders a compact design and low price of the camera.